High energy density capacitors

ABSTRACT

Capacitors comprising an inert porous shaped body onto which a first electrically conductive layer, a second layer of barium titanate and a further electrically conductive layer have been applied.

The present invention relates to capacitors comprising an inert porousshaped body onto which a first electrically conductive layer, a secondlayer of barium titanate and a further electrically conductive layerhave been applied.

Capacitors perform many tasks in information technology and electricenergy engineering. There has in recent times been a search forcapacitors which have a high energy density and can perform the task ofbatteries or be used for covering short-term high load requirements.

Electrochemica Acta 45 (2000), 2483 to 2498, discloses electrochemicalor double-layer capacitors. These devices, also known as supercapacitorsor ultracapacitors, store electric energy in two capacitors which areconnected in series and each have an electric double layer which isformed between the two electrodes and the ions in the electrolyte. Thedistance in which charge separation occurs is only a few Angstrom. Aselectrolytes, use is made of highly porous carbon having internalsurface areas of up to 2500 m²/g. As indicated by the capacitor formulaC=E ₀ ·E·A/dwhere C is the capacitance, E₀ is the absolute dielectric constant, E isthe dielectric constant of the dielectric, A is the area of thecapacitor and d is the distance between the electrodes, capacitances ofup to 100 farad/cm³ are possible at large areas A and small spacings d.

Such double-layer capacitors (supercapacitors) at present achieve energydensities of from 3 to 7 Wh/kg or Wh/liter, which are far below theenergy densities of conventional batteries (lithium ion batteriesachieve from 150 to 200 Wh/kg). This is due to the maximum possiblevoltage loading being restricted to about 3.5 V by the electrochemicalstability of the electrolyte.

On the other hand, there is a type of capacitor which operates at highvoltages, namely ceramic capacitors comprising dielectrics based onbarium titanate.

Ceramic capacitors which comprise dielectrics based on barium titanateand operate at high working voltages because of the high dielectricbreakdown resistance of the titanates of up to 200 V/0.1 μm are knownfrom the prior art. However, ceramic capacitors have relatively lowcapacitances.

It is an object of the present invention to remedy the abovementioneddisadvantages.

We have found that this object is achieved by new and improvedcapacitors which comprise an inert porous shaped body onto which a firstelectrically conductive layer, a second layer of barium titanate and afurther electrically conductive layer have been applied.

The capacitors of the present invention can be produced as follows:

An inert porous shaped body can, in a first step, be provided with afirst electrically conductive layer and this can be provided with acontact. A second layer of barium titanate can be applied on top of thefirst layer and, finally, another electrically conductive layer can beapplied on top of this titanate layer and be provided with a contact.The capacitors obtained in this way can be hermetically sealed, e.g.encapsulated, except for the electric contacts.

Suitable porous shaped bodies are in general catalyst support materials,for example those based on metal oxides such as aluminum oxide, silicondioxide, titanium dioxide, zirconium dioxide, chromium oxide or mixturesthereof, preferably aluminum oxide, silicon dioxide, titanium dioxide,zirconium dioxide or mixtures thereof, particularly preferably aluminumoxide, zirconium dioxide or mixtures thereof, or carbides, preferablysilicon carbide, having a BET-surface area of from 0.1 to 20 m²/g,preferably from 0.5 to 10 m²/g, particularly preferably from 1 to 5m²/g, a pore content of from 10 to 90% by volume, preferably from 30 to85% by volume, particularly preferably from 50 to 80% by volume, andpore sizes of from 0.01 to 100 μm, preferably from 0.1 to 30 μm,particularly preferably from 1 to 10 μm.

The shaped bodies can have any shapes, for example rings, pellets,stars, wagon wheels, honeycombs, preferably cuboids, cylinders,rectangles or boxes of generally any size (diameter, longest edgelength). In the case of capacitors for information technology, forexample, the size is generally in the range from 1 to 10 mm. Largerdimensions are necessary in energy engineering.

To produce the first conductive layer on the shaped body, metals such ascopper, nickel, chromium or mixtures thereof can be applied in any layerthickness, generally from 10 nm to 1000 nm, preferably from 50 nm to 500nm, particularly preferably from 100 nm to 200 nm.

The application of the electrically conductive layer to the shaped bodycan be carried out using all known methods such as vapor deposition,sputtering or electroless plating, preferably electroless plating. Inelectroless plating, the shaped bodies are infiltrated or impregnatedwith suitable, commercially available plating liquids and heated totemperatures below 100° C. to deposit the metal. After metal deposition,the liquid, usually water, can be removed at elevated temperatures and,if desired, under reduced pressure.

It is also possible, for example in the case of iron or nickel, toproduce the first conductive layer by heating the shaped bodies in ironcarbonyl or nickel carbonyl vapors. In the case of iron, the shapedbodies can be heated to from about 150 to 200° C., and in the case ofnickel to from 50 to 100° C.

In a preferred embodiment, the shaped bodies can be heated to elevatedtemperatures of from 50 to 100° C. in an inert atmosphere (e.g. nitrogenor argon) to produce a homogeneous metal layer. It may be advantageousto apply crystallization nuclei, e.g. nuclei based on platinum metals,likewise by impregnation with suitable liquids (see above).

Finally, the first metal layer can be provided with a contact. This canbe carried out, for example, by soldering a metal foil onto an area ofthe metal-coated shaped body (production of the first electrode).

A dielectric can then be applied on top of the initially producedelectrode. This is advantageously carried out using dispersions ofcrystalline titanate particles having sizes of less than 10 nm inalcohols. Such dispersions can be prepared by reaction of titaniumalkoxides with barium hydroxides or strontium hydroxides in alcoholicsolution as described in the German application No.: 102 21 499.9 (O.Z.0050/53537).

The shaped body can be infiltrated or impregnated with such a dispersionwhich may contain from 5 to 60% by weight, preferably from 10 to 40% byweight, of titanate particles, followed by removal of the alcohol byincreasing the temperature to 30-100° C., preferably 50-80° C., and, ifdesired, reducing the ambient pressure to deposit the titanium particleson the first electrode.

To produce a homogeneous, dense layer of the dielectric, the shapedbodies can be heated to from 700 to 1200° C., preferably from 900 to1100° C., in an inert gas atmosphere so that the titanate particlessinter together to form a dense film.

To increase the layer thickness, the impregnation with the titanatedispersion and the sintering can be repeated a number of times. Thelayer thickness is generally from 10 to 1000 nm, preferably from 20 to500 nm, particularly preferably from 100 to 300 nm.

Finally, a second electrode layer can be applied in a manner analogousto that employed for the first.

After the second electrode layer has been applied, this can be providedwith a contact on the side opposite the first contact, thus producingthe capacitor. The latter can be hermetically encapsulated to protect itand for the purposes of insulation.

The capacitors of the present invention are suitable as smoothingcapacitors or energy storing capacitors or phase shift capacitors inelectric energy engineering and as coupling capacitors, filtercapacitors or miniature energy storage capacitors in informationtechnology.

The capacitors of the present invention may be illustrated as follows:

A specific surface area (BET surface area) of the porous shaped body of2 m²/g and a barium titanate layer thickness of 0.1 μm at a relativedielectric constant of 5000 (“The Effect of Grain Size on the DielectricProperties of Barium Titanate Ceramic”, A. J. Bell and A. J. Moulson, inElectrical Ceramics, British Ceramic Proceedings No. 36, October 1985,pages 57-65) gives a capacitance calculated according to the formula onpage 1, line 29, of about 1 farad/cm³. Such a capacitor can be chargedto a voltage of 200 V, and its energy density is then 20 000 Ws/cm³ orapproximately 5.5 kWh/liter.

1. A capacitor comprising an inert porous shaped body onto which a firstelectrically conductive layer, a layer of barium titanate and a secondelectrically conductive layer have been applied.
 2. A capacitor asclaimed in claim 1 consisting of an inert porous shaped body onto whicha first electrically conductive layer, a layer of barium titanate and asecond electrically conductive layer have been applied.
 3. A capacitoras claimed in claim 1, wherein the BET surface area of the inert porousshaped body is from 0.1 to 20 m²/g.
 4. A capacitor as claimed in claim1, wherein the pore content of the inert, porous shaped body is from 10to 90% by volume.
 5. A process for producing said capacitor as claimedin claim 1, said process comprising applying a first electricallyconductive layer onto an inert porous shaped body, applying a layer ofbarium titanate on top of said electrically conductive layer andapplying a second electrically conductive layer on top of said layer ofbarium titanate.
 6. (canceled)
 7. The capacitor as claimed in claim 1,wherein said capacitor is a smoothing capacitor, energy storagecapacitor, phase shift capacitor, coupling capacitor, filter capacitoror miniature energy storage capacitor.